The present invention relates to a method of manufacturing a semiconductor used in light-emitting devices such as a light-emitting diode device or a semiconductor laser diode device having wavelengths in the region from blue to ultraviolet, and more specifically to a method of manufacturing a gallium nitride semiconductor which has excellent electric and optical properties by using a vapor-phase growth process.
Recently, short-wave light-emitting devices which have wavelengths shorter than blue light have been promising as a light source for full-color display or an optical disk capable of performing high-density recording. Such short-wave light-emitting devices have been vigorously studied by utilizing semiconductors which employ II-IV family compounds such as zinc selenide (ZnSe), IV family compounds such as silicon carbide (SiC), and III-V family compounds such as gallium nitride (GaN). Since blue light-emitting diodes which employ GaN, GaInN, or the like among the III-V family compounds have been realized, light-emitting devices which utilize these gallium nitride semiconductors have been drawing attention.
In order to grow crystals in gallium nitride semiconductors, a metalorganic vapor-phase epitaxy process (MOVPE) and a molecular beam epitaxy process (MBE) are generally used. When the single crystal of GaN is grown by a vapor-phase process, either SiC or sapphire (Al.sub.2 O.sub.3) is used as the substrate.
In the case where SiC is used as the substrate, a SiC substrate consisting of 6H polycrystal-type and having a (0001) surface is used. As shown in FIG. 10, SiC has a lattice mismatching rate of as small as about 3% with GaN, and has a much smaller lattice mismatching rate of 1% with aluminum nitride (AlN). For this reason, SiC has become promising especially in these days as the substrate of a nitride compound semiconductor. Unlike sapphire, SiC has a conductivity, which enables a SiC substrate to have an electrode on the back side thereof. Consequently, a light-emitting device such as a laser device can be obtained by a simple process.
According to the above-mentioned MOVPE, trimethylaluminum (TMA), which is metalorganic, and ammonia (NH.sub.3) are supplied onto a SiC substrate by using hydrogen (H.sub.2) as a carrier gas, and as a result, a single crystal consisting of AlN is grown as a buffer layer on the substrate at a temperature of about 1000.degree. C. Then, the supply of TMA is suspended and in turn trimethylgallium (TMG) is supplied onto the substrate with a temperature fixed at 1000.degree. C., thereby a single crystal consisting of GaN being grown on the buffer layer. The buffer layer consisting of AlN most generally has a thickness of about 50 nm, and has not been doped, so that it has a high resistance and a low conductivity. It has been reported that the dislocation density of GaN on the buffer layer on the SiC substrate in this case is 10.sup.9 cm.sup.-2.
However, the conventional method of manufacturing a gallium nitride semiconductor has the following problem. When a semiconductor crystal layer which composes a device such as a clad layer or an active layer grows on the buffer layer which has grown on the SiC substrate, the semiconductor crystal layer suffers cracks.